The present invention relates to solid state microdosimetry and more particularly to devices and methodology concerning a passive array of microstructure radiation sensitive volumes which enable the recording of radiation exposures occurring on a microscopic level.
The probability that a given exposure to low-level ionizing radiation will result in significant damage to an organism depends on the number of ionizations generated within the regions of biological cells containing DNA, principally the cell nucleus. As a result, the National Council on Radiation Protection and Measurement defines, for example, dose equivalent (DE) limits for work exposure to a specific type of radiation in terms of the product of the dose, a measure of the number of ionizations per unit volume expressed in terms of energy deposition per unit mass, and a quality factor (QF) which depends on the density of ionizations along the particle's trajectory. See ICRP "Recommendations of the International Commission on Radiological Protection," ICRP Publication 60, Annals of the ICRP, 21 No. 1-3, Pergamon Press, Oxford, 1991. That is: EQU DE(rem)=QF.times.Dose(rads) (Equation 1)
Dose equivalent is not, however, the only measurement of the propensity of radiation to damage a given type of irradiated area or volume. Other measurements have been employed in performing such a spectral analysis of incident radiation. In particular, as detailed in the above-referenced ICRP Publication, the ICRP has defined dose equivalent related concepts of equivalent dose and effective dose. For ease of explanation, however, dose equivalent will be hereafter given as the primary example.
Ionization density is normally expressed in terms of the charged particle's linear energy transfer (LET). Particles with higher LET values deposit relatively more energy and generate more ionizations within the biological cell nuclei they traverse. The probability of a somatic mutation or other biological effect increases with the LET of the incident radiation until some optimum value and then falls off at higher LET values. The relationship among dose, dose equivalent, and cell response is described in more detail in the scholarly articles by C.A. Sondhaus, V. P. Bond, and L. E. Feinendegen, "Cell Oriented Alternatives to Dose, Quality Factor, and Dose Equivalent for Low-Level Radiation," Health Physics 59, 35-48 (1990); V. P. Bond and M. N. Varma, "Low-Level Radiation Response Explained in Terms of Fluence and Cell Critical Volume Dose" and "Empirical Evaluation of Cell Critical Volume Dose vs. Cell Response Function for Pink Mutations in Tradescantia," in Eight Symposium on Microdosimetry, (Julich, Germany, Commission of the European Communities, 1983) 423-450.
An instrument capable of measuring the energy deposition in small microvolumes and assigning quality factors to each event separately is disclosed in commonly assigned U.S. Pat. No. 5,256,879. The disclosure of such '879 patent is hereby fully incorporated herein by reference. The invention disclosed in the '879 patent is an active microdosimetry device, i.e., one requiring associated electronics and a constant power source. More particularly, it records transient events and requires its associated electronics to store a permanent record of radiation exposure.
While the disclosure of the '879 patent is appropriate and fully satisfactory in many instances, many other applications for a microdosimeter, however, favor a totally passive device (i.e., one requiring no power during exposure). Such is particularly true, for example, with respect to personnel radiation detection devices and space applications. Personnel detectors preferably should be small and light enough to be comfortable and to allow freedom of movement. The absence of a power source and measurement electronics as associated with active microdosimeters therefore makes passive devices relatively more attractive in such situations. Similarly, the size, weight, and power consumption constraints involved in any application for use in space make a passive microdosimeter approach more attractive than an active microdosimeter arrangement.
As a result, a choice between an active microdosimeter and prior passive dosimeter arrangements requires a choice between accepting the above-described constraints involved with an active microdosimeter and the inability of typical previous passive dosimeters to distinguish among radiation that is likely to cause damage in biological cells or microelectronic devices.
Therefore, to avoid the necessity of choosing between only such two devices, it is desirable to have a passive microdosimetry device, that is, a device capable of calculating dose-equivalent or similar measure of the propensity of radiation to damage an irradiated area of interest and having an array large enough to measure exposure levels as low as a few millirem, yet which requires no individual detector-associated power source and measurement circuitry. Such a device would be able, for example, to detect and distinguish between events generated by neutrons and/or alpha particles. It would also be beneficial if the device were inexpensive and have simple on-board instrumentation. Current state of the art arrangements are generally described below.
There are generally two types of radiation detection instruments: dosimeters and microdosimeters. Dosimeters measure exposure in terms of dose. Microdosimeters characterize exposure in terms of dose equivalent or similar measurement capable of describing the propensity of incident radiation to damage an irradiated volume. Both types of instrumentation can be further characterized according to whether they are active or passive, i.e., according to whether they require power while recording exposure. A device is active if power is so required; it is passive if not.
Dosimeters generally characterize radiation exposure in terms of rads (ergs/gram), which is the dose, or the energy deposited per unit mass. Dose, in turn, is proportional to the number of ionizations per unit volume within a given material. As is explained more fully in U.S. Pat. No. 5,256,879 referenced above, dosimeters generally do not distinguish events according to the type of radiation and are limited to measuring exposure in terms of the amount of energy deposited per unit volume (dose) and the rate at which that energy is deposited (dose rate).
Dosimeters can be, furthermore, divided into active and passive devices. The passive devices cumulatively record some effect of the exposure which when the device is "read" can be translated into dose. That is, passive devices do not require power to record events during exposure to the incident radiation field. The radiation events leave a lasting effect upon the non-powered devices which a measurement device can later read or which causes some visible or audible effect upon the device. Passive devices may include various items such as film badges and thermo-luminescent dosimeter (TLD) devices.
In contrast, active devices require some type of external power to detect a radiation event. They may, for example, be used as integrating devices, such as a pocket dosimeter using an ionization chamber or a p-n diode, to measure total dose. They may also be continuously monitored to determine the dose rate as well as the total integrated dose. In particular, the latter configuration may be connected to a circuit which provides an audible and/or visible warning of dangerous levels of dose rate.
One type of active dosimeter employs a RadFET. The RadFET device is described in some detail in A. G. Holmes-Siedle, L. Adams, N. G. Blamires, and D. H. J. Totterdel, "PMOS Dosimeters: Long Term Annealing and Neutron Response", IEEE Transactions on Nuclear Science NS-33, 1310 (1986). A RadFET dosimeter incorporates a single transistor which is relatively large in size. As is generally true of all dosimeters, therefore, the RadFET device exhibits a large sensitive volume. For a metal oxide semiconductor, the sensitive volume may be generally defined as that volume about the junction within which charges (electron/hole pairs) generated by traversing radiation particles are efficiently collected at the junction.
As is discussed in more detail in the Detailed Description below, the likelihood that incident radiation will damage, for example, a cell nucleus or a DNA genome depends upon the size of the cell or genome. Thus, to qualitatively analyze an incident radiation field as to its propensity to cause such damage, the sensitive volume of the radiation detector should approximate the size of the physical volume of interest. As a result of its relatively large sensitive volume, therefore, the RadFet device is incapable of providing a radiation analysis compatible with ICRP weighting factors.
Although the circuitry used to read the RadFET dosimeter may vary, a basic method underlying the device is to measure the turn-on voltage of a PMOS transistor, that is, the voltage which must be applied between the source and drain of the PMOS transistor to turn it on. Exposure to radiation changes this turn-on voltage and, therefore, the RadFET may be used as a dosimeter by monitoring such change as caused by an incident field.
In particular, ionizing radiation causes a build-up of charge at the interface between the oxide and the substrate under the gate. The charge build-up is approximately proportional to the amount of energy deposited (number of electron-hole pairs generated) within the oxide under the gate. The gate is maintained at a constant voltage determined by the circuit and the methodology followed in reading the dosimeter. The charge deposited as a result of the radiation, therefore, moves the turn-on voltage either closer to, or farther away from, the charge held on the gate, depending on the device configuration. The energy deposition in the device may then be determined by measuring the difference between gate voltage and the post-irradiation turn-on voltage and comparing this measure to the pre-irradiation difference.
Dosimeters using such technology typically use a single transistor per sensor. Dosimeters based on such design, as is true of dosimeters generally, fail to distinguish among types of radiation. As a result, the RadFET technology as currently used is incapable of monitoring exposure in terms of dose equivalent or similar measurements.
As is described above, microdosimeters characterize exposure in terms of dose equivalent or similar measurement capable of describing the propensity of incident radiation to damage an irradiated volume. Such devices generally accomplish such a spectral analysis through the use of microstructure sensitive volumes that approximate the size of, for example, biological cell nuclei, DNA genomes, or micro-electronic junctions. One example of an active microdosimeter is the device disclosed in the above-referenced U.S. Pat. No. 5,256,879. Another example of an active device is the gas microdosimeter. Although such latter type of device employs a relatively large area within which radiation events are analyzed, microstructure areas are approximated by varying gas density within the device. Due to size and cost constraints, gas microdosimeters are not generally practical in, for example, personnel detection applications.